High-voltage capacitors are useful in technology applications where brief high-voltage pulses must be delivered, for example, in automatic implantable cardioverter/defibrillators ("ICDs") where high-voltage pulses are required across the defibrillation lead system to affect defibrillation or cardioversion. An ICD and package therefor, including rolled (or "wound") capacitors, are described in U.S. Pat. No. 4,254,775 to Langer.
Typically, electrolytic capacitors are used in these applications because they have the best properties in terms of energy density and ability to withstand relatively high voltage. Aluminum electrolytic capacitors are generally used, having aluminum foil rolled into a small volume in the form of a cylinder. By etching the surface of the aluminum foil prior to rolling, the surface area and the capacitance are increased accordingly. In the case of such rolled or cylindrical aluminum electrolytic capacitor, the foil is etched, then formed to provide an oxide layer which functions as the dielectric for the capacitor. The formed foil is rolled and then "aged" in the presence of an electrolyte to grow oxide on any exposed aluminum. Aging is the process of slowly increasing the voltage on the capacitor after impregnation with electrolyte over the course of many hours by charging the capacitor using a small current source. After reaching the maximum rated voltage, the voltage may be decreased and the temperature increased. Further details of the construction of such traditional high voltage capacitors used in ICDs are described by P. J. Troup, "Implantable Cardioverters and Defibrillators," at pp. 704-713 (Current Problems in Cardiology, Vol. XIV, No. 12, December 1989, Year Book Medical Publishers, Chicago), which pages are incorporated herein by reference.
In implantable defibrillators, as in other applications where space is a critical design element it is desirable to use capacitors with the greatest possible capacitance per unit volume. One problem with cylindrical aluminum electrolytic capacitors is that the foil cannot be etched as much as might be desirable because heavily etched and formed foils are very fragile and may break during the rolling step. Additionally, the cylindrical shape does not provide an optimum device for packaging of the ICD. One way to allow increased etching of the foil and thus increase capacitance per unit area is to construct the capacitor in a flat, layered or stacked configuration with very highly etched anode foil. This also allows the ICD designer to select the general profile of the capacitor. An implantable cardiac defibrillator with improved flat capacitors is described in U.S. Pat. No. 5,131,388 to Pless et al., which patent is incorporated herein by reference.
For layered capacitors, after the foil is etched, voltage is applied to the foil through an electrolyte such as boric acid or citric acid and water or other solutions familiar to those skilled in the state of the art, resulting in the formation of aluminum oxide (Al.sub.2 O.sub.3) on the surface of the anode foil. Following formation of the oxide on the foil, individual anode plates are punched, stamped or otherwise cut out of the foil in a shape to conform to the capacitor package. The cut edges around the periphery of the anode plates are carefully cleaned to remove particulates of anode material that can get caught between the capacitor plates in the stacked configuration resulting in a high leakage current or capacitor failure. After assembling the capacitor by stacking anode plates, paper and cathode foil, the cut edges of the plates and any other exposed aluminum are then reformed in the capacitor during the aging process to reduce leakage current. However, the oxide produced at the edges during reforming is not as good a quality as the original oxide due to process differences.
It is therefore an object of the present invention to provide a capacitor anode foil plate that does not suffer from the above drawbacks.
It is another object of the invention to provide a method of making a capacitor anode foil plate having a high quality oxide layer at the plate edges.
It is still another object of the invention provide an anode plate which improves the manufacturability and performance of layered aluminum electrolytic capacitors.